Some food products tend to ferment rather rapidly after the containers in which they are packaged have been opened and sterility has been lost by exposure to air. This is especially apt to happen with fruit juices, for example. The fermentation evolves gases which, if the container closure is resealed, cause internal gas pressure to increase, especially if there is relatively little "head space" above the level of the food product in the container. Apart from fermentation, excessively high internal gas pressure in a container can arise in other ways, for example if the container is overheated while sealed. In extreme cases such internal pressure can reach a level so high as to rupture and shatter a container. In the packaging industry internal gas pressures in excess of 40 pounds are considered undesirably high.
For this reason there has been a need for a closure which will effectively seal and reseal a container against ordinary internal gas pressures but which will automatically vent or release a gas pressure which exceeds some predetermined limiting value.